Method of forming a low voltage gate oxide layer and tunnel oxide layer in an EEPROM cell

ABSTRACT

A method of fabricating a non-volatile memory embedded logic circuit having a low voltage logic gate oxide layer and tunnel oxide layer is described. Both the low voltage logic gate oxide and the tunnel oxide layers are formed in a single step, thereby reducing the number of overall processing steps needed to form the devices.

CROSS-REFERENCE TO RELATED APPLICATION

This is a divisional of pending U.S. patent application Ser. No.10/717,149 filed Nov. 18, 2003.

TECHNICAL FIELD

The present invention relates generally to the non-volatile memorydevices and more particularly to a non-volatile memory embedded logicdevice.

BACKGROUND ART

Non-volatile memory cells such as EEPROM cells typically have adouble-layer polycrystalline silicon (“poly”) structure that includes acontrol gate layer and a floating gate layer. In contrast, semiconductorlogic gates, having a control gate only, require only a singlepolysilicon process to form the control gate layer. To improve computingspeed and reduce device size, non-volatile memory cells are sometimesembedded into logic chips. Since processes for forming a non-volatilememory cell and a logic gate are quite different, they are traditionallyformed in a separate series of steps.

To reduce a total number of processing steps for a non-volatile memoryembedded logic circuit, it is often desirable to form the embeddednon-volatile memory cells using a single-layer poly structure. FIG. 1Ashows a cross-section of a typical single-layered EEPROM cell 10dissected along a wordline. FIG. 1B shows a cross-section of the sameEEPROM cell 10 dissected along a bitline. With reference to FIG. 1A, aP-channel single poly EEPROM cell 10 is formed in an N-well 14 providedwithin a P-substrate. With reference to FIG. 1B, the EEPROM cell 10includes a P-channel select transistor 24 and a P channel storagetransistor 26. A first P+ diffusion region 28 serves as both a drain forstorage transistor 26 and a source for select transistor 24, and asecond P+ diffusion region 30, which is coupled to a bitline 36, servesas a drain for select transistor 24. The single-layer polysilicon 20serves as a floating gate for the storage transistor 26 and a selectgate for the select transistor 24. Referring back to FIG. 1A, anapplication of a bias voltage to a control gate 12 enhances a channel 22(FIG. 1B) extending between a source 32 and the drain 28 of storagetransistor 26, and an application of a bias voltage to the select gate24 enhances a channel 34 between the source 28 and the drain 30 ofselect transistor 24.

Referring again to FIG. 1A, a P-type buried diffusion layer serves asthe control gate 12 for the EEPROM cell 10. A layer of silicon oxide 18that is approximately 350 Å thick is provided between the floating gate20 and the control gate 12. A tunnel oxide layer 16 that is about 70 Åthick lies between the floating gate 20 and the N-well 14. Thesingle-poly silicon EEPROM cell 10 is programmed, erased, and read in amanner similar to that of a double-poly silicon cell. That is,programming is accomplished by electron tunneling from the floating gate20 to the substrate 14 through the tunnel oxide 16 while erasing isrealized by electrons tunneling from the substrate 14 to the floatinggate 20.

Although the single poly silicon process described above allows theformation of a single polysilicon layer for both the floating gates ofnon-volatile memory cells and the control gates of the logic cells inthe same step, the oxide layer underneath the polysilicon layer has tobe formed in separate steps because its thickness varies throughout theembedded circuit. For instance, the thickness of a typical gate oxidelayer for a low voltage logic gate is approximately 130 Å for 5Vsystems, 50 Å for 2.5V systems and 30 Å for 1.8V systems. On the otherhand, the tunnel oxide layer and the oxide layer between the floatinggate and the control gate of a EEPROM cell is typically around 70 Åthick. Because the oxide layer thickness of the logic cells and theEEPROM cells are so different, they are typically formed inseparate-steps. For instance, U.S. Pat. No. 6,238,979 to Bergemontteaches an embedment of EEPROM cells in a logic device by forming theEEPROM cells first, followed by masking the completed EEPROM cells toform logic gates. It would be desirable to have an embedded circuitstructure and a method for forming the structure that would allow theformation of the oxide layer for both the logic gate and thenon-volatile memory cell in one step, thereby eliminating the need toform the EEPROM cells and the logic gates separately.

DISCLOSURE OF THE INVENTION

The present invention teaches the formation of a non-volatile memoryembedded logic circuit having three types of active areas: one for thenon-volatile memory cells, one for low voltage logic gates, and one forhigh voltage logic gates. The low voltage logic gate and thenon-volatile memory cell having an oxide layer of essentially the samethickness while the high voltage logic gate has an oxide layer that isthicker. The embedded memory structure disclosed herein allows theforming of the non-volatile memory gate oxide layer and the logic gateoxide layer in a single step, thereby reducing manufacturing time.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A shows a cross section of a prior art single-polysiliconnon-volatile memory dissected along a wordline.

FIG. 1B shows a cross section of the prior art single-polysiliconnon-volatile memory dissected along a bit line.

FIG. 2A shows a top view of a single-polysilicon EEPROM cell accordingto one exemplary embodiment of the present invention.

FIG. 2B shows a cross-sectional view of the single-polysilicon EEPROMcell shown in FIG. 2A taken along line A-A.

FIG. 2C shows a cross-sectional view of the single-polysilicon EEPROMcell shown in FIG. 2A taken along line B-B, a cross-sectional view of alow voltage logic gate cell, and a high voltage logic gate cell of thepresent invention.

FIG. 3A shows the step-by-step formation of a low voltage logic gate anda high voltage logic gate according to the preferred embodiment of thepresent invention.

FIG. 3B shows the step-by-step formation of a single-polysiliconnon-volatile memory cell according to the preferred embodiment of thepresent invention.

MODES FOR CARRYING OUT THE INVENTION

In FIG. 2A, a top view of an EEPROM cell 100 according to an exemplaryembodiment of the present invention is shown. A cross-sectional view ofthe EEPROM cell 100 taken along the wordline (segmenting line A-A) isshown in FIG. 2B. A cross-sectional view of the EEPROM cell 100 takenalong the bitline (segmenting line B-B) is shown alongside a highvoltage logic gate 96 and a low voltage logic gate 94 in FIG. 2C.

With reference to FIGS. 2A-2C, the EEPROM cell 100 is made up of aburied control gate 80, a floating gate 82, and a tunneling region 84comprised of a tunneling extension 86 below the floating gate 82. Thecontrol gate 80 is connected to a voltage source through a tungsten plug88. There are basically two oxide layers throughout the whole embeddedcircuit: a thick oxide layer 90 for the high voltage logic gate 96 andthe select gate 98, and a thinner oxide layer 92 for a tunnel oxide ofthe EEPROM cell 100 and the low voltage logic gate 94.

Exemplary process steps for simultaneously forming the EEPROM cell 100and the logic gates 94, 96, 98 are shown in FIGS. 3A and 3B. FIG. 3Ashows the steps for making a high voltage MOS transistor and a lowvoltage MOS transistor. FIG. 3B shows the steps for making an EEPROMcell as seen in a cross-sectional view taken along a wordline. Althoughnot shown in the figures for purposes of brevity and clarity, it will beunderstood that various other processing steps may be performed inbetween the steps shown. These steps may include, for example, ionimplantation steps to form various N-well and P-well regions and theformation of tungsten plugs to the control gates.

In step (i), a pad oxide layer 44 is formed above a semiconductorsubstrate 46. A nitride layer 42 is deposited on top of the pad oxidelayer 44. Next, a patterned photoresist layer 40 is formed above thenitride layer 42. The nitride layer 42, pad oxide layer 44, andsubstrate 46 are etched, so as to produce an exposed area 50 where anisolation structure is to be formed.

The isolation structure surrounds and electrically isolates individualdevice areas in which logic cells and embedded memory cells are built.Though the isolation structure shown in subsequent figures is of theShallow Trench Isolation (STI) type, it is also possible to use otherisolation methods such as Local Oxidation of Silicon (LOCOS). In an STIprocess, the isolation structure is formed by etching a shallow trenchthrough a nitride layer 42 and pad oxide 44 and into an exposedsubstrate area, for example to a depth of about 4000 Å, and then filledor thermally grown, for example with silicon dioxide, using a depositionor growth process according to methods known to one skilled in the art.In step (ii), a shallow trench 48 is formed by an etch of the exposedarea 50, followed by an oxide fill step that fills the shallow trenchwith, for example, silicon dioxide. A subsequent planarization process,using the nitride layer 42 as a natural stop, removes or polishes offany excess silicon dioxide material, forming a leveled oxide plateau 49on top of the STI structure 48. As an example, the polishing step may bea chemical mechanical planarization (CMP) process.

In step (iii), the nitride layer 42 and the pad oxide layer 44 aresequentially removed to form an STI isolation structure 48 shown. TheSTI isolation structure is formed to electrically separate adjacentdevice areas. A variety of adjacent and non-adjacent device structuresmay now be formed.

Next, as shown in FIGS. 3A and B at step (iv), a high voltage (HV) gateoxide layer 68, for example having an approximate 250 Å thickness, isformed over the substrate 46. The oxide layer 68 is masked with apatterned photoresist. The exposed part of the oxide layer 68 is thenetched to reveal portions of the underlying substrate 46 as shown instep (v). Next, in exposed areas 52, 54, a second or subsequent, thinneroxide layer will be formed. The formation of the first or second oxidelayer may be carried out by thermal oxidation of the substrate, chemicalvapor deposition, or atomic layer deposition. As shown in step (vi), afirst device area will be used to form an EEPROM tunnel oxide layer 58,a second device area will be used to form a high voltage logic gateoxide layer 67, and a third device area will be used to form a lowvoltage logic gate oxide layer 56. Referring to FIG. 2B, an N+region maybe developed in the EEPROM cell area to form a control gate 80. The LVgate oxide layer 56 and the tunnel oxide layer 58 each, for example,approximately 70 Å thick, have been formed on the exposed portions 52,54 of the substrate 46. This layer of thin gate oxide 56, 58 serves as agate oxide 56 for the low voltage (LV) logic gate and a tunnel oxide 58for the EEPROM cell respectively. The low voltage logic gate oxide layer56 may have essentially the same thickness as the tunnel oxide layer 58.

Thereafter, as shown in Figure (vii), a polysilicon layer 64, 66 isdeposited on top of the oxide layers 56, 58, 67, and 68 to form acontrol gate layer 64 over the gate oxide layer 56 in the logic gatearea and a floating gate layer 66 over the tunnel oxide layer 58 in theEEPROM cell area respectively.

Although the present invention has been described in terms of specificexemplary embodiments, one skilled in the art will realize that otherembodiments may be readily envisioned that are still the presentinvention. Therefore, the present invention shall be limited in scopeonly by the appended claims.

1. A non-volatile memory embedded logic circuit comprising: a firstdevice area, a second device area, and a third device area; a firstoxide layer on top of said second device area functioning as a gateoxide for a high voltage logic gate; a second oxide layer that isthinner than said first oxide layer on top of said first and thirddevice areas, whereby said second oxide layer on top of said firstdevice area functions as a tunnel oxide while said second oxide layer ontop of said third device area functions as a gate oxide for a lowvoltage logic gate; and a floating gate layer overlying said first oxidelayer.
 2. The circuit of claim 1, wherein said first oxide layer isapproximately 250 Å thick.
 3. The non-volatile memory embedded logiccircuit of claim 1, wherein said second oxide layer is approximately 70Å thick.
 4. The non-volatile memory embedded logic circuit of claim 1,wherein said floating gate layer is a doped polysilicon layer.
 5. Thenon-volatile memory embedded logic circuit of claim 1, wherein saidfirst oxide layer is formed in one processing step.
 6. The non-volatilememory embedded logic circuit of claim 1, wherein said second oxidelayer is formed in one processing step.
 7. A non-volatile memoryembedded logic circuit comprising: a first device area and a seconddevice area; a first oxide layer on top of said second device areafunctioning as a gate oxide for a high voltage logic gate; a secondoxide layer that is thinner than said first oxide layer on top of saidfirst device area, whereby said second oxide layer on top of said firstdevice area functions as a tunnel oxide; and a floating gate layeroverlying said first oxide layer.
 8. The circuit of claim 7, whereinsaid first oxide layer is approximately 250 Å thick.
 9. The non-volatilememory embedded logic circuit of claim 7, wherein said second oxidelayer is approximately 70 Åthick.